Purpose of Staging in Turbines

Why turbines have stages?

What is a steam turbine stage?

- a stage is a pair of rotating and stationary blades (diaphragm, nozzle)
- the rotating blades are attached to the rotor
- the stationary blades (stators) are attached to the turbine casing
- steam pressure and temperature drops as it expands and passes to the stages
- steam expansion means increase in volume meaning larger surface area

Why do steam turbines have multiple stages? Why not just use a single stage turbine?

- one stage extracts only a small portion of the available energy from the steam
- more stages means more energy extracted
- more energy means more mechanical work at the shaft
- more work means more power (shaft horsepower)

Why turbines need higher vacuum (lower pressure)?

- higher condenser vacuum pressure (lower pressure) means lower exhaust temperature of the steam
- higher vacuum pressure means higher thermal efficiency
- higher efficiency means higher power output
- higher efficiency means lesser power losses

Pressure is inversely proportional to Area

HIGH PRESSURE (HP) TURBINE
- steam is at Higher pressure
- steam is at Higher temperature
- steam is at Higher speed
- turbine has Lesser area (lesser diameter)

LOW PRESSURE (LP) TURBINE
- steam is at Lower pressure
- steam is at Lower temperature
- steam is at Lower speed
- turbine has Larger area (larger diameter)